modeling cube into cylinder
I took the liberty to simplify the provided answers.




  Leon Cheung-method:


Bevel Ctrl+B edges and use Clamp Overlap C while beveling.

Example:






  stacker-method:


Enable LoopTools Addon that comes with Blender.

Then, use: W > LoopTools > Circle - Even though sometimes you need to manually adjust some angles.

Example: